This invention relates to sizing agents for carbon fibers.
Carbon fiber reinforced plastics (CFRP), which are composites of carbon fibers with a matrix resin such as epoxy resins, unsaturated polyester resins and polyamide resins, are among the most desirable materials from the point of view of specific modulus and specific strength and are rapidly coming to be used in aerospace and other industries because of their superior qualities and light weight. Carbon fibers which are used for the production of CFRP are unidirected into the form of filaments or tows and are combined with a matrix resin after they are made into strands or sheets, woven or knit. Since carbon fibers are basically a brittle substance which does not elongate easily, fluffs tend to be produced by mechanical friction during processing if they are used directly without any pretreatment. Difficulty in their handling becomes a problem and the mechanical properties of CFRP obtained therefrom are also adversely affected. It is therefore a common practice to coat such carbon fibers with a sizing agent in order to improve their cohesiveness and abrasion resistance. This invention relates to sizing agents for carbon fibers to be combined with an unsaturated polyester matrix resin.
Examples of prior art sizing agent for coating carbon fibers to be combined with an unsaturated polyester matrix resin include those which use epoxidized polybutadiene (Japanese Patent Publication Tokkai 56-4335), those which use a mixture of bisphenol diglycidyl ether and a prepolymer derived from diallyl phthalate (Japanese Patent Publication Tokkai 59-228083), those of a water emulsion type having as indispensable constituents an epoxy resin, a condensation product of an unsaturated dibasic acid and oxyalkylated bisphenol, and oxyalkylene derivative of phenol (U.S. Pat. No. 4,167,538) and those using an unsaturated epoxy compound as coupling agent (U.S. Pat. No. 4,163,003).
Although these prior art sizing agents have their own advantages, they still involve serious problems related to the production of CFRP as well as their mechanical properties such as fluffs and yarn breakage at the time of weaving because of the low cohesiveness and abrasion resistance of carbon fibers, danger of toxicity and flammability because an organic solvent is used, and low interlaminar shear strength (ILSS) of produced CFRP because of poor adhesiveness between carbon fibers and unsaturated polyester matrix resins.